In electrostatography, image charge patterns are formed on a support and are developed by treatment with an electrostatographic developer containing marking particles which are attracted to the charge patterns. These particles are called toner particles or, collectively, toner. The image charge pattern, also referred to as an electrostatic latent image, is formed on an insulative surface of an electrostatographic element by any of a variety of methods. For example, the electrostatic latent image may be formed electrophotographically, by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate. Alternatively, the electrostatic latent image may be formed by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are employed in cascade and magnetic brush electrostatographic development processes. The toner particles and carrier particles differ triboelectrically, such that during mixing to form the developer, the toner particles acquire a charge of one polarity and the carrier particles acquire a charge of the opposite polarity. The opposite charges cause the toner particles to cling to the carrier particles. During development, the electrostatic forces of the latent image, sometimes in combination with an additional applied field, attract the toner particles. The toner particles are pulled away from the carrier particles and become electrostatically attached, in imagewise relation, to the latent image bearing surface. The resultant toner image can then be fixed, by application of heat or other known methods, depending upon the nature of the toner image and the surface, or can be transferred to another surface and then fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited on the latent image-bearing surface.
Toner particles in many dry, two-component electrostatographic developers include a charge control agent. The charge control agent desirably, provides a high uniform net electrical charge to toner particles without reducing the adhesion of the toner to paper or other medium. Positive charge control agents, materials which impart a positive charge to toner particles in a developer, have been widely used and a great many are described in the published patent literature. In contrast, choices are more limited with negative charge control agents and negatively charging toners.
Carrier particles comprise a core material coated with a polymer. Commonly used polymers include: silicone resin; acrylic polymers, such as, poly(methylmethacrylate); and vinyl polymers, such as polystyrene and combinations of materials. Another commonly used coating material is fluorohydrocarbon polymer, such as poly(vinylidene fluoride) or poly(vinylidene fluoride-co-tetrafluoroethylene). See, for example, U.S. Pat. Nos. 4,546,060; 4,478,925; 4,076,857; and 3,970,571. Such polymeric fluorohydrocarbon carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material, in order to adjust the degree of triboelectric charging of both the carrier and the toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces, such as photoconductive element surfaces, that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as "scumming"). A further purpose can be to alter the electrical resistance of the carrier particles. All of these, and even more, purposes are well known in the art for polymeric fluorohydrocarbon carrier coatings.
While such carrier coatings can serve all of the above-noted purposes well, in some cases the coatings do not adequately serve some or all of these purposes simultaneously. For example, in some developer compositions, polymeric fluorohydrocarbon carrier coatings can serve many of the above-noted purposes well, but, depending upon the nature of the toner particles and carrier core material desired to be included in the developer, such carrier coatings can cause the developer to acquire a triboelectric charge that is too high for optimum developer performance; that is, the electrostatic latent image has difficulty pulling the toner particles away from the carrier particles.
Various methods have been used to improve the characteristics of fluorocarbon coated carriers. U.S. Pat. No. 4,737,435 to Yoerger, disclosed a method of dehydrofluorinating a fluorohydrocarbon carrier coating by contacting the coated carrier particles with a basic solution. The resulting change in chemical structure had the effect of repositioning the carrier triboelectrically. U.S. Pat. No. 4,726,994 to Yoerger, disclosed a method of dehydrofluorinating and oxidizing a fluorohydrocarbon carrier coating by contacting the coated carrier particles with a basic solution and with an oxidizing agent. The resulting change in chemical structure also had the effect of repositioning the carrier triboelectrically, and in addition, decreased overcharging. Examples in both U.S. Pat. Nos. 4,737,435 and 4,726,994 are directed to positively charging developer (referring to the charge on the toner particles), that is, developer which contains negatively charging carrier.
There is a continuing need for negatively charging developers (developers in which toner charges negatively and carrier positively). The methods of U.S. Pat. Nos. 4,737,435 and 4,726,994 provide beneficial results when applied to negatively charging developers, however, those results are not optimal in all circumstances. For example, treatment with base and oxidizer may result in a toner charge that is excessively high. There is thus a continuing need for convenient methods for treating carrier particles to improve charging characteristics, which are applicable to negatively charging carrier.